<!doctype html>
<html>
<head>
<meta charset='UTF-8'><meta name='viewport' content='width=device-width initial-scale=1'>

<link href='https://fonts.loli.net/css?family=Open+Sans:400italic,700italic,700,400&subset=latin,latin-ext' rel='stylesheet' type='text/css' /><style type='text/css'>html {overflow-x: initial !important;}:root { --bg-color:#ffffff; --text-color:#333333; --select-text-bg-color:#B5D6FC; --select-text-font-color:auto; --monospace:"Lucida Console",Consolas,"Courier",monospace; --title-bar-height:20px; }
.mac-os-11 { --title-bar-height:28px; }
html { font-size: 14px; background-color: var(--bg-color); color: var(--text-color); font-family: "Helvetica Neue", Helvetica, Arial, sans-serif; -webkit-font-smoothing: antialiased; }
body { margin: 0px; padding: 0px; height: auto; bottom: 0px; top: 0px; left: 0px; right: 0px; font-size: 1rem; line-height: 1.42857; overflow-x: hidden; background: inherit; tab-size: 4; }
iframe { margin: auto; }
a.url { word-break: break-all; }
a:active, a:hover { outline: 0px; }
.in-text-selection, ::selection { text-shadow: none; background: var(--select-text-bg-color); color: var(--select-text-font-color); }
#write { margin: 0px auto; height: auto; width: inherit; word-break: normal; overflow-wrap: break-word; position: relative; white-space: normal; overflow-x: visible; padding-top: 36px; }
#write.first-line-indent p { text-indent: 2em; }
#write.first-line-indent li p, #write.first-line-indent p * { text-indent: 0px; }
#write.first-line-indent li { margin-left: 2em; }
.for-image #write { padding-left: 8px; padding-right: 8px; }
body.typora-export { padding-left: 30px; padding-right: 30px; }
.typora-export .footnote-line, .typora-export li, .typora-export p { white-space: pre-wrap; }
.typora-export .task-list-item input { pointer-events: none; }
@media screen and (max-width: 500px) {
  body.typora-export { padding-left: 0px; padding-right: 0px; }
  #write { padding-left: 20px; padding-right: 20px; }
  .CodeMirror-sizer { margin-left: 0px !important; }
  .CodeMirror-gutters { display: none !important; }
}
#write li > figure:last-child { margin-bottom: 0.5rem; }
#write ol, #write ul { position: relative; }
img { max-width: 100%; vertical-align: middle; image-orientation: from-image; }
button, input, select, textarea { color: inherit; font: inherit; }
input[type="checkbox"], input[type="radio"] { line-height: normal; padding: 0px; }
*, ::after, ::before { box-sizing: border-box; }
#write h1, #write h2, #write h3, #write h4, #write h5, #write h6, #write p, #write pre { width: inherit; }
#write h1, #write h2, #write h3, #write h4, #write h5, #write h6, #write p { position: relative; }
p { line-height: inherit; }
h1, h2, h3, h4, h5, h6 { break-after: avoid-page; break-inside: avoid; orphans: 4; }
p { orphans: 4; }
h1 { font-size: 2rem; }
h2 { font-size: 1.8rem; }
h3 { font-size: 1.6rem; }
h4 { font-size: 1.4rem; }
h5 { font-size: 1.2rem; }
h6 { font-size: 1rem; }
.md-math-block, .md-rawblock, h1, h2, h3, h4, h5, h6, p { margin-top: 1rem; margin-bottom: 1rem; }
.hidden { display: none; }
.md-blockmeta { color: rgb(204, 204, 204); font-weight: 700; font-style: italic; }
a { cursor: pointer; }
sup.md-footnote { padding: 2px 4px; background-color: rgba(238, 238, 238, 0.7); color: rgb(85, 85, 85); border-radius: 4px; cursor: pointer; }
sup.md-footnote a, sup.md-footnote a:hover { color: inherit; text-transform: inherit; text-decoration: inherit; }
#write input[type="checkbox"] { cursor: pointer; width: inherit; height: inherit; }
figure { overflow-x: auto; margin: 1.2em 0px; max-width: calc(100% + 16px); padding: 0px; }
figure > table { margin: 0px; }
tr { break-inside: avoid; break-after: auto; }
thead { display: table-header-group; }
table { border-collapse: collapse; border-spacing: 0px; width: 100%; overflow: auto; break-inside: auto; text-align: left; }
table.md-table td { min-width: 32px; }
.CodeMirror-gutters { border-right: 0px; background-color: inherit; }
.CodeMirror-linenumber { user-select: none; }
.CodeMirror { text-align: left; }
.CodeMirror-placeholder { opacity: 0.3; }
.CodeMirror pre { padding: 0px 4px; }
.CodeMirror-lines { padding: 0px; }
div.hr:focus { cursor: none; }
#write pre { white-space: pre-wrap; }
#write.fences-no-line-wrapping pre { white-space: pre; }
#write pre.ty-contain-cm { white-space: normal; }
.CodeMirror-gutters { margin-right: 4px; }
.md-fences { font-size: 0.9rem; display: block; break-inside: avoid; text-align: left; overflow: visible; white-space: pre; background: inherit; position: relative !important; }
.md-fences-adv-panel { width: 100%; margin-top: 10px; text-align: center; padding-top: 0px; padding-bottom: 8px; overflow-x: auto; }
#write .md-fences.mock-cm { white-space: pre-wrap; }
.md-fences.md-fences-with-lineno { padding-left: 0px; }
#write.fences-no-line-wrapping .md-fences.mock-cm { white-space: pre; overflow-x: auto; }
.md-fences.mock-cm.md-fences-with-lineno { padding-left: 8px; }
.CodeMirror-line, twitterwidget { break-inside: avoid; }
.footnotes { opacity: 0.8; font-size: 0.9rem; margin-top: 1em; margin-bottom: 1em; }
.footnotes + .footnotes { margin-top: 0px; }
.md-reset { margin: 0px; padding: 0px; border: 0px; outline: 0px; vertical-align: top; background: 0px 0px; text-decoration: none; text-shadow: none; float: none; position: static; width: auto; height: auto; white-space: nowrap; cursor: inherit; -webkit-tap-highlight-color: transparent; line-height: normal; font-weight: 400; text-align: left; box-sizing: content-box; direction: ltr; }
li div { padding-top: 0px; }
blockquote { margin: 1rem 0px; }
li .mathjax-block, li p { margin: 0.5rem 0px; }
li blockquote { margin: 1rem 0px; }
li { margin: 0px; position: relative; }
blockquote > :last-child { margin-bottom: 0px; }
blockquote > :first-child, li > :first-child { margin-top: 0px; }
.footnotes-area { color: rgb(136, 136, 136); margin-top: 0.714rem; padding-bottom: 0.143rem; white-space: normal; }
#write .footnote-line { white-space: pre-wrap; }
@media print {
  body, html { border: 1px solid transparent; height: 99%; break-after: avoid; break-before: avoid; font-variant-ligatures: no-common-ligatures; }
  #write { margin-top: 0px; padding-top: 0px; border-color: transparent !important; }
  .typora-export * { -webkit-print-color-adjust: exact; }
  .typora-export #write { break-after: avoid; }
  .typora-export #write::after { height: 0px; }
  .is-mac table { break-inside: avoid; }
  .typora-export-show-outline .typora-export-sidebar { display: none; }
}
.footnote-line { margin-top: 0.714em; font-size: 0.7em; }
a img, img a { cursor: pointer; }
pre.md-meta-block { font-size: 0.8rem; min-height: 0.8rem; white-space: pre-wrap; background: rgb(204, 204, 204); display: block; overflow-x: hidden; }
p > .md-image:only-child:not(.md-img-error) img, p > img:only-child { display: block; margin: auto; }
#write.first-line-indent p > .md-image:only-child:not(.md-img-error) img { left: -2em; position: relative; }
p > .md-image:only-child { display: inline-block; width: 100%; }
#write .MathJax_Display { margin: 0.8em 0px 0px; }
.md-math-block { width: 100%; }
.md-math-block:not(:empty)::after { display: none; }
.MathJax_ref { fill: currentcolor; }
[contenteditable="true"]:active, [contenteditable="true"]:focus, [contenteditable="false"]:active, [contenteditable="false"]:focus { outline: 0px; box-shadow: none; }
.md-task-list-item { position: relative; list-style-type: none; }
.task-list-item.md-task-list-item { padding-left: 0px; }
.md-task-list-item > input { position: absolute; top: 0px; left: 0px; margin-left: -1.2em; margin-top: calc(1em - 10px); border: none; }
.math { font-size: 1rem; }
.md-toc { min-height: 3.58rem; position: relative; font-size: 0.9rem; border-radius: 10px; }
.md-toc-content { position: relative; margin-left: 0px; }
.md-toc-content::after, .md-toc::after { display: none; }
.md-toc-item { display: block; color: rgb(65, 131, 196); }
.md-toc-item a { text-decoration: none; }
.md-toc-inner:hover { text-decoration: underline; }
.md-toc-inner { display: inline-block; cursor: pointer; }
.md-toc-h1 .md-toc-inner { margin-left: 0px; font-weight: 700; }
.md-toc-h2 .md-toc-inner { margin-left: 2em; }
.md-toc-h3 .md-toc-inner { margin-left: 4em; }
.md-toc-h4 .md-toc-inner { margin-left: 6em; }
.md-toc-h5 .md-toc-inner { margin-left: 8em; }
.md-toc-h6 .md-toc-inner { margin-left: 10em; }
@media screen and (max-width: 48em) {
  .md-toc-h3 .md-toc-inner { margin-left: 3.5em; }
  .md-toc-h4 .md-toc-inner { margin-left: 5em; }
  .md-toc-h5 .md-toc-inner { margin-left: 6.5em; }
  .md-toc-h6 .md-toc-inner { margin-left: 8em; }
}
a.md-toc-inner { font-size: inherit; font-style: inherit; font-weight: inherit; line-height: inherit; }
.footnote-line a:not(.reversefootnote) { color: inherit; }
.md-attr { display: none; }
.md-fn-count::after { content: "."; }
code, pre, samp, tt { font-family: var(--monospace); }
kbd { margin: 0px 0.1em; padding: 0.1em 0.6em; font-size: 0.8em; color: rgb(36, 39, 41); background: rgb(255, 255, 255); border: 1px solid rgb(173, 179, 185); border-radius: 3px; box-shadow: rgba(12, 13, 14, 0.2) 0px 1px 0px, rgb(255, 255, 255) 0px 0px 0px 2px inset; white-space: nowrap; vertical-align: middle; }
.md-comment { color: rgb(162, 127, 3); opacity: 0.8; font-family: var(--monospace); }
code { text-align: left; vertical-align: initial; }
a.md-print-anchor { white-space: pre !important; border-width: initial !important; border-style: none !important; border-color: initial !important; display: inline-block !important; position: absolute !important; width: 1px !important; right: 0px !important; outline: 0px !important; background: 0px 0px !important; text-decoration: initial !important; text-shadow: initial !important; }
.md-inline-math .MathJax_SVG .noError { display: none !important; }
.html-for-mac .inline-math-svg .MathJax_SVG { vertical-align: 0.2px; }
.md-fences-math .MathJax_SVG_Display, .md-math-block .MathJax_SVG_Display { text-align: center; margin: 0px; position: relative; text-indent: 0px; max-width: none; max-height: none; min-height: 0px; min-width: 100%; width: auto; overflow-y: visible; display: block !important; }
.MathJax_SVG_Display, .md-inline-math .MathJax_SVG_Display { width: auto; margin: inherit; display: inline-block !important; }
.MathJax_SVG .MJX-monospace { font-family: var(--monospace); }
.MathJax_SVG .MJX-sans-serif { font-family: sans-serif; }
.MathJax_SVG { display: inline; font-style: normal; font-weight: 400; line-height: normal; text-indent: 0px; text-align: left; text-transform: none; letter-spacing: normal; word-spacing: normal; overflow-wrap: normal; white-space: nowrap; float: none; direction: ltr; max-width: none; max-height: none; min-width: 0px; min-height: 0px; border: 0px; padding: 0px; margin: 0px; zoom: 90%; }
#math-inline-preview-content { zoom: 1.1; }
.MathJax_SVG * { transition: none 0s ease 0s; }
.MathJax_SVG_Display svg { vertical-align: middle !important; margin-bottom: 0px !important; margin-top: 0px !important; }
.os-windows.monocolor-emoji .md-emoji { font-family: "Segoe UI Symbol", sans-serif; }
.md-diagram-panel > svg { max-width: 100%; }
[lang="flow"] svg, [lang="mermaid"] svg { max-width: 100%; height: auto; }
[lang="mermaid"] .node text { font-size: 1rem; }
table tr th { border-bottom: 0px; }
video { max-width: 100%; display: block; margin: 0px auto; }
iframe { max-width: 100%; width: 100%; border: none; }
.highlight td, .highlight tr { border: 0px; }
mark { background: rgb(255, 255, 0); color: rgb(0, 0, 0); }
.md-html-inline .md-plain, .md-html-inline strong, mark .md-inline-math, mark strong { color: inherit; }
.md-expand mark .md-meta { opacity: 0.3 !important; }
mark .md-meta { color: rgb(0, 0, 0); }
@media print {
  .typora-export h1, .typora-export h2, .typora-export h3, .typora-export h4, .typora-export h5, .typora-export h6 { break-inside: avoid; }
}
.md-diagram-panel .messageText { stroke: none !important; }
.md-diagram-panel .start-state { fill: var(--node-fill); }
.md-diagram-panel .edgeLabel rect { opacity: 1 !important; }
.md-require-zoom-fix foreignobject { font-size: var(--mermaid-font-zoom); }
.md-fences.md-fences-math { font-size: 1em; }
.md-fences-math .MathJax_SVG_Display { margin-top: 8px; cursor: default; }
.md-fences-advanced:not(.md-focus) { padding: 0px; white-space: nowrap; border: 0px; }
.md-fences-advanced:not(.md-focus) { background: inherit; }
.typora-export-show-outline .typora-export-content { max-width: 1440px; margin: auto; display: flex; flex-direction: row; }
.typora-export-sidebar { width: 300px; font-size: 0.8rem; margin-top: 80px; margin-right: 18px; }
.typora-export-show-outline #write { --webkit-flex:2; flex: 2 1 0%; }
.typora-export-sidebar .outline-content { position: fixed; top: 0px; max-height: 100%; overflow: hidden auto; padding-bottom: 30px; padding-top: 60px; width: 300px; }
@media screen and (max-width: 1024px) {
  .typora-export-sidebar, .typora-export-sidebar .outline-content { width: 240px; }
}
@media screen and (max-width: 800px) {
  .typora-export-sidebar { display: none; }
}
.outline-content li, .outline-content ul { margin-left: 0px; margin-right: 0px; padding-left: 0px; padding-right: 0px; list-style: none; }
.outline-content ul { margin-top: 0px; margin-bottom: 0px; }
.outline-content strong { font-weight: 400; }
.outline-expander { width: 1rem; height: 1.42857rem; position: relative; display: table-cell; vertical-align: middle; cursor: pointer; padding-left: 4px; }
.outline-expander::before { content: ""; position: relative; font-family: Ionicons; display: inline-block; font-size: 8px; vertical-align: middle; }
.outline-item { padding-top: 3px; padding-bottom: 3px; cursor: pointer; }
.outline-expander:hover::before { content: ""; }
.outline-h1 > .outline-item { padding-left: 0px; }
.outline-h2 > .outline-item { padding-left: 1em; }
.outline-h3 > .outline-item { padding-left: 2em; }
.outline-h4 > .outline-item { padding-left: 3em; }
.outline-h5 > .outline-item { padding-left: 4em; }
.outline-h6 > .outline-item { padding-left: 5em; }
.outline-label { cursor: pointer; display: table-cell; vertical-align: middle; text-decoration: none; color: inherit; }
.outline-label:hover { text-decoration: underline; }
.outline-item:hover { border-color: rgb(245, 245, 245); background-color: var(--item-hover-bg-color); }
.outline-item:hover { margin-left: -28px; margin-right: -28px; border-left: 28px solid transparent; border-right: 28px solid transparent; }
.outline-item-single .outline-expander::before, .outline-item-single .outline-expander:hover::before { display: none; }
.outline-item-open > .outline-item > .outline-expander::before { content: ""; }
.outline-children { display: none; }
.info-panel-tab-wrapper { display: none; }
.outline-item-open > .outline-children { display: block; }
.typora-export .outline-item { padding-top: 1px; padding-bottom: 1px; }
.typora-export .outline-item:hover { margin-right: -8px; border-right: 8px solid transparent; }
.typora-export .outline-expander::before { content: "+"; font-family: inherit; top: -1px; }
.typora-export .outline-expander:hover::before, .typora-export .outline-item-open > .outline-item > .outline-expander::before { content: "−"; }
.typora-export-collapse-outline .outline-children { display: none; }
.typora-export-collapse-outline .outline-item-open > .outline-children, .typora-export-no-collapse-outline .outline-children { display: block; }
.typora-export-no-collapse-outline .outline-expander::before { content: "" !important; }
.typora-export-show-outline .outline-item-active > .outline-item .outline-label { font-weight: 700; }


.CodeMirror { height: auto; }
.CodeMirror.cm-s-inner { background: inherit; }
.CodeMirror-scroll { overflow: auto hidden; z-index: 3; }
.CodeMirror-gutter-filler, .CodeMirror-scrollbar-filler { background-color: rgb(255, 255, 255); }
.CodeMirror-gutters { border-right: 1px solid rgb(221, 221, 221); background: inherit; white-space: nowrap; }
.CodeMirror-linenumber { padding: 0px 3px 0px 5px; text-align: right; color: rgb(153, 153, 153); }
.cm-s-inner .cm-keyword { color: rgb(119, 0, 136); }
.cm-s-inner .cm-atom, .cm-s-inner.cm-atom { color: rgb(34, 17, 153); }
.cm-s-inner .cm-number { color: rgb(17, 102, 68); }
.cm-s-inner .cm-def { color: rgb(0, 0, 255); }
.cm-s-inner .cm-variable { color: rgb(0, 0, 0); }
.cm-s-inner .cm-variable-2 { color: rgb(0, 85, 170); }
.cm-s-inner .cm-variable-3 { color: rgb(0, 136, 85); }
.cm-s-inner .cm-string { color: rgb(170, 17, 17); }
.cm-s-inner .cm-property { color: rgb(0, 0, 0); }
.cm-s-inner .cm-operator { color: rgb(152, 26, 26); }
.cm-s-inner .cm-comment, .cm-s-inner.cm-comment { color: rgb(170, 85, 0); }
.cm-s-inner .cm-string-2 { color: rgb(255, 85, 0); }
.cm-s-inner .cm-meta { color: rgb(85, 85, 85); }
.cm-s-inner .cm-qualifier { color: rgb(85, 85, 85); }
.cm-s-inner .cm-builtin { color: rgb(51, 0, 170); }
.cm-s-inner .cm-bracket { color: rgb(153, 153, 119); }
.cm-s-inner .cm-tag { color: rgb(17, 119, 0); }
.cm-s-inner .cm-attribute { color: rgb(0, 0, 204); }
.cm-s-inner .cm-header, .cm-s-inner.cm-header { color: rgb(0, 0, 255); }
.cm-s-inner .cm-quote, .cm-s-inner.cm-quote { color: rgb(0, 153, 0); }
.cm-s-inner .cm-hr, .cm-s-inner.cm-hr { color: rgb(153, 153, 153); }
.cm-s-inner .cm-link, .cm-s-inner.cm-link { color: rgb(0, 0, 204); }
.cm-negative { color: rgb(221, 68, 68); }
.cm-positive { color: rgb(34, 153, 34); }
.cm-header, .cm-strong { font-weight: 700; }
.cm-del { text-decoration: line-through; }
.cm-em { font-style: italic; }
.cm-link { text-decoration: underline; }
.cm-error { color: red; }
.cm-invalidchar { color: red; }
.cm-constant { color: rgb(38, 139, 210); }
.cm-defined { color: rgb(181, 137, 0); }
div.CodeMirror span.CodeMirror-matchingbracket { color: rgb(0, 255, 0); }
div.CodeMirror span.CodeMirror-nonmatchingbracket { color: rgb(255, 34, 34); }
.cm-s-inner .CodeMirror-activeline-background { background: inherit; }
.CodeMirror { position: relative; overflow: hidden; }
.CodeMirror-scroll { height: 100%; outline: 0px; position: relative; box-sizing: content-box; background: inherit; }
.CodeMirror-sizer { position: relative; }
.CodeMirror-gutter-filler, .CodeMirror-hscrollbar, .CodeMirror-scrollbar-filler, .CodeMirror-vscrollbar { position: absolute; z-index: 6; display: none; }
.CodeMirror-vscrollbar { right: 0px; top: 0px; overflow: hidden; }
.CodeMirror-hscrollbar { bottom: 0px; left: 0px; overflow: hidden; }
.CodeMirror-scrollbar-filler { right: 0px; bottom: 0px; }
.CodeMirror-gutter-filler { left: 0px; bottom: 0px; }
.CodeMirror-gutters { position: absolute; left: 0px; top: 0px; padding-bottom: 30px; z-index: 3; }
.CodeMirror-gutter { white-space: normal; height: 100%; box-sizing: content-box; padding-bottom: 30px; margin-bottom: -32px; display: inline-block; }
.CodeMirror-gutter-wrapper { position: absolute; z-index: 4; background: 0px 0px !important; border: none !important; }
.CodeMirror-gutter-background { position: absolute; top: 0px; bottom: 0px; z-index: 4; }
.CodeMirror-gutter-elt { position: absolute; cursor: default; z-index: 4; }
.CodeMirror-lines { cursor: text; }
.CodeMirror pre { border-radius: 0px; border-width: 0px; background: 0px 0px; font-family: inherit; font-size: inherit; margin: 0px; white-space: pre; overflow-wrap: normal; color: inherit; z-index: 2; position: relative; overflow: visible; }
.CodeMirror-wrap pre { overflow-wrap: break-word; white-space: pre-wrap; word-break: normal; }
.CodeMirror-code pre { border-right: 30px solid transparent; width: fit-content; }
.CodeMirror-wrap .CodeMirror-code pre { border-right: none; width: auto; }
.CodeMirror-linebackground { position: absolute; left: 0px; right: 0px; top: 0px; bottom: 0px; z-index: 0; }
.CodeMirror-linewidget { position: relative; z-index: 2; overflow: auto; }
.CodeMirror-wrap .CodeMirror-scroll { overflow-x: hidden; }
.CodeMirror-measure { position: absolute; width: 100%; height: 0px; overflow: hidden; visibility: hidden; }
.CodeMirror-measure pre { position: static; }
.CodeMirror div.CodeMirror-cursor { position: absolute; visibility: hidden; border-right: none; width: 0px; }
.CodeMirror div.CodeMirror-cursor { visibility: hidden; }
.CodeMirror-focused div.CodeMirror-cursor { visibility: inherit; }
.cm-searching { background: rgba(255, 255, 0, 0.4); }
span.cm-underlined { text-decoration: underline; }
span.cm-strikethrough { text-decoration: line-through; }
.cm-tw-syntaxerror { color: rgb(255, 255, 255); background-color: rgb(153, 0, 0); }
.cm-tw-deleted { text-decoration: line-through; }
.cm-tw-header5 { font-weight: 700; }
.cm-tw-listitem:first-child { padding-left: 10px; }
.cm-tw-box { border-style: solid; border-right-width: 1px; border-bottom-width: 1px; border-left-width: 1px; border-color: inherit; border-top-width: 0px !important; }
.cm-tw-underline { text-decoration: underline; }
@media print {
  .CodeMirror div.CodeMirror-cursor { visibility: hidden; }
}


:root {
    --side-bar-bg-color: #fafafa;
    --control-text-color: #777;
}

@include-when-export url(https://fonts.loli.net/css?family=Open+Sans:400italic,700italic,700,400&subset=latin,latin-ext);

/* open-sans-regular - latin-ext_latin */
  /* open-sans-italic - latin-ext_latin */
    /* open-sans-700 - latin-ext_latin */
    /* open-sans-700italic - latin-ext_latin */
  html {
    font-size: 16px;
}

body {
    font-family: "Open Sans","Clear Sans", "Helvetica Neue", Helvetica, Arial, sans-serif;
    color: rgb(51, 51, 51);
    line-height: 1.6;
}

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</style><title>01UI视图</title>
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<body class='typora-export os-windows'><div class='typora-export-content'>
<div id='write'  class=''><h2 id='一ui视图'><span>一.UI视图</span></h2><h3 id='11-uitableview相关'><span>1.1 UITableView相关</span></h3><p><strong><span>A.Cell的重用机制</span></strong>
<code>UITableView</code><span>通过重用Cell来达到节省内存，提高性能的目的，它的实现过程是，当屏幕上的Cell数量超越屏幕高度，滑动</span><code>UITableView</code><span>时，系统会将滑出屏幕外的Cell放到重用队列中，当有新的Cell即将要滑入屏幕时，就会先从重用队列中查询是否有可以重用的Cell，如果有，就拿过来用，如果没有就创建一个来使用。</span></p><p><em><span>扩展</span></em><span>：重用机制，虽然很简单，但是日常工作中，却可以大量应用，比如模型的复用：当页面的refresh下拉刷新，我们一般是将当前dataArray清空掉，重新创建模型，但是其实我们可以新建一个缓存数组，来重复使用这些模型对象，提高内存的使用效率。</span></p><h3 id='12-ui数据源同步'><span>1.2 UI数据源同步</span></h3><p><span>问题描述：在子线程网络请求的过程中，用户执行在主线程的删除操作，引发的请求网络后刷新UI而删除数据还存在的问题？</span></p><p><span>答：这个问题的本质是，多线程对数组的操作，线程同步问题，有很多解决办法。</span>
<span>比如</span><code>YYKit</code><span>里面使用线程安全的数组(</span><code>YYThreadSafeArray</code><span>)，它的思路是封装</span><code>NSMutableArray</code><span>，使用GCD信号量</span><code>dispatch_semaphore_t</code><span>的办法，对数组的增删等操作时进行加锁。由于读写操作都使用了同一个信号量进行控制，可以得知此方案对可变数组的多线程操作是串行的，可以保证可变数组在多线程下访问的安全。（读和写，不能同时）。</span></p><p><span>也可以使用GCD的</span><code>dispatch_barrier_async</code><span>栅栏方案实现一个多读单写的数组。实现在写入时，不能有其他操作。读取的时候可以并行读取。</span></p><p><span>总而言之，线程安全会牺牲一些性能，具体场景问题，具体分析即可。</span></p><pre class="md-fences md-end-block ty-contain-cm modeLoaded" spellcheck="false" lang=""><div class="CodeMirror cm-s-inner CodeMirror-wrap" lang=""><div style="overflow: hidden; position: relative; width: 3px; height: 0px; top: 9.5px; left: 8px;"><textarea autocorrect="off" autocapitalize="off" spellcheck="false" tabindex="0" style="position: absolute; bottom: -1em; padding: 0px; width: 1000px; height: 1em; outline: none;"></textarea></div><div class="CodeMirror-scrollbar-filler" cm-not-content="true"></div><div class="CodeMirror-gutter-filler" cm-not-content="true"></div><div class="CodeMirror-scroll" tabindex="-1"><div class="CodeMirror-sizer" style="margin-left: 0px; margin-bottom: 0px; border-right-width: 0px; padding-right: 0px; padding-bottom: 0px;"><div style="position: relative; top: 0px;"><div class="CodeMirror-lines" role="presentation"><div role="presentation" style="position: relative; outline: none;"><div class="CodeMirror-measure"><span><span>​</span>x</span></div><div class="CodeMirror-measure"></div><div style="position: relative; z-index: 1;"></div><div class="CodeMirror-code" role="presentation" style=""><div class="CodeMirror-activeline" style="position: relative;"><div class="CodeMirror-activeline-background CodeMirror-linebackground"></div><div class="CodeMirror-gutter-background CodeMirror-activeline-gutter" style="left: 0px; width: 0px;"></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;">线程同步问题，可以参考使用GCD，不过会降低性能。</span></pre></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;">https://www.jianshu.com/p/ac99295585a1</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span cm-text="">​</span></span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;">YYKit里面的例子`YYThreadSafeArray`:</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;">https://www.jianshu.com/p/9082d66fff3c</span></pre></div></div></div></div></div><div style="position: absolute; height: 0px; width: 1px; border-bottom: 0px solid transparent; top: 115px;"></div><div class="CodeMirror-gutters" style="display: none; height: 115px;"></div></div></div></pre><h3 id='13-ui事件源传递响应'><span>1.3 UI事件源传递&amp;响应</span></h3><p><strong><span>A.UIView和CALayer的关系</span></strong>
<span>1.从框架来说：</span><code>CALayer</code><span>属于</span><code>QuartzCore</code><span>框架，</span><code>UIView</code><span>属于</span><code>UIKit</code><span>框架.</span>
<span>2.从功能来说：</span></p><ul><li><span>2.1 UIView为CALayer提供内容，负责处理触摸等事件，参与响应链</span></li><li><span>2.2 CALayer负责显示内容contents的渲染、动画等</span></li></ul><p><span>3.从继承关系来分析两者：UIView : UIResponder，CALayer : NSObject</span>
<span>4.从设计上讲，体现了设计原则里面的单一职责.</span></p><p><span>详细参考：</span><a href='https://www.jianshu.com/p/e5cc76bf800b' target='_blank' class='url'>https://www.jianshu.com/p/e5cc76bf800b</a></p><p><strong><span>B.UI事件传递与视图响应链</span></strong></p><p><img src="image/0056.jpeg" referrerpolicy="no-referrer" alt="alt 图像显示原理" title="图像显示原理"></p><p><span>先是事件的传递，然后是事件的响应链，两部分：</span>
<span>1.事件的传递：</span>
<span>在iOS中，能够响应事件的对象都是</span><code>UIResponder</code><span>的子类对象。</span>
<span>1.1 当手指操作屏幕时，会产生</span><code>UITouch</code><span>对象和</span><code>UIEvent</code><span>对象。</span>
<span>1.2 这两个对象产生后会被传递到</span><code>UIApplication</code><span>管理的一个事件队列中</span>
<span>1.3 App的</span><code>UIWindow</code><span>会在视图层次结构中找到一个最合适的View来处理触摸事件，也就是事件会从父控件传给子控件，从</span><code>UIWindow</code><span>到</span><code>UIView</code><span>的过程</span>
<span>1.4 事件从父控件传给子控件的过程中，关键方法是</span><code>hitTest:withEvent:</code>
<span>1.5 如果没有找到合适的子view，那么就会将UIWindow作为响应者</span></p><p><span>2.开始视图响应链：</span>
<span>2.1 通过事件传递，找到了响应的View，判断view是否能响应处理事件，不能则继续往上</span><code>superView</code>
<span>2.2 如果view的控制器存在，就传递给控制器处理；如果控制器不存在，则传递给它的父视图</span>
<span>2.2 在视图层次结构的最顶层，如果也不能处理收到的事件，则将事件传递给UIWindow对象进行处理</span>
<span>2.3 如果UIWindow对象也不能处理，则将事件传递给UIApplication对象</span>
<span>2.4 如果UIApplication也不能处理该事件，则将该事件丢弃</span></p><p><span>3.需要注意的几点：</span>
<span>3.1 事件的传递过程中，查找合适view的两个核心方法是：</span></p><pre class="md-fences md-end-block ty-contain-cm modeLoaded" spellcheck="false" lang=""><div class="CodeMirror cm-s-inner CodeMirror-wrap" lang=""><div style="overflow: hidden; position: relative; width: 3px; height: 0px; top: 9.5px; left: 8px;"><textarea autocorrect="off" autocapitalize="off" spellcheck="false" tabindex="0" style="position: absolute; bottom: -1em; padding: 0px; width: 1000px; height: 1em; outline: none;"></textarea></div><div class="CodeMirror-scrollbar-filler" cm-not-content="true"></div><div class="CodeMirror-gutter-filler" cm-not-content="true"></div><div class="CodeMirror-scroll" tabindex="-1"><div class="CodeMirror-sizer" style="margin-left: 0px; margin-bottom: 0px; border-right-width: 0px; padding-right: 0px; padding-bottom: 0px;"><div style="position: relative; top: 0px;"><div class="CodeMirror-lines" role="presentation"><div role="presentation" style="position: relative; outline: none;"><div class="CodeMirror-measure"><pre><span>xxxxxxxxxx</span></pre></div><div class="CodeMirror-measure"></div><div style="position: relative; z-index: 1;"></div><div class="CodeMirror-code" role="presentation"><div class="CodeMirror-activeline" style="position: relative;"><div class="CodeMirror-activeline-background CodeMirror-linebackground"></div><div class="CodeMirror-gutter-background CodeMirror-activeline-gutter" style="left: 0px; width: 0px;"></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;">hitTest:withEvent:</span></pre></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;">pointInside:withEvent:</span></pre></div></div></div></div></div><div style="position: absolute; height: 0px; width: 1px; border-bottom: 0px solid transparent; top: 46px;"></div><div class="CodeMirror-gutters" style="display: none; height: 46px;"></div></div></div></pre><p><span>3.2 在递归遍历子View的过程中，是采用的倒序遍历，最后添加的view，最先被遍历</span>
<span>3.3 在事件响应链中，判断判断一个对象是否能响应处理这个事件，核心方法是：</span><code>toouchXX</code><span>等几个方法。</span>
<span>3.4 只有继承了UIResponder的对象才能接受并处理事件，我们称之为“响应者对象”。而几个</span><code>toouchXX</code><span>方法其实就是</span><code>UIResponder</code><span>类里面的方法。</span></p><p><span>详情：</span><a href='https://www.jianshu.com/p/32f8c16c90ac' target='_blank' class='url'>https://www.jianshu.com/p/32f8c16c90ac</a></p><p><strong><span>B1.事件的传递和响应的区别：</span></strong>
<span>事件的传递: 是从上到下（父控件到子控件）</span>
<span>事件的响应: 是从下到上（顺着响应者链条向上传递：子控件到父控件)</span></p><ul><li><span>1、当一个事件发生后，事件会从父控件传给子控件，也就是说由UIApplication -&gt; UIWindow -&gt; UIView -&gt; initial view</span>
<span>以上就是事件的传递，也就是寻找最合适的view的过程。</span></li><li><span>2、接下来是事件的响应。首先看initial view能否处理这个事件，如果不能则会将事件传递给其上级视图（inital view的superView）；如果上级视图仍然无法处理则会继续往上传递；一直传递到视图控制器viewController，首先判断视图控制器的根视图view是否能处理此事件；如果不能则接着判断该视图控制器能否处理此事件，如果还是不能则继续向上传递；</span></li><li><span>2.1（对于第二个图视图控制器本身还在另一个视图控制器中，则继续交给父视图控制器的根视图，如果根视图不能处理则交给父视图控制器处理）；一直到window，如果window还是不能处理此事件则继续交给Application处理，如果最后application还是不能处理此事件则将其丢弃</span></li><li><span>3、在事件的响应中，如果某个控件实现了</span><code>touchesXX</code><span>方法，则这个事件将由该控件来接受，如果调用了</span><code>[super touchesXX]</code><span>;就会将事件顺着响应者链条往上传递，传递给下一个响应者；接着就会调用下一个响应者的</span><code>touchesXX</code><span>方法</span></li></ul><p><span>如何做到一个事件多个对象处理：</span>
<span>因为系统默认做法是把事件上抛给父控件，所以可以通过重写自己的</span><code>touches</code><span>方法和父控件的</span><code>touches</code><span>方法来达到一个事件多个对象处理的目的。</span></p><pre class="md-fences md-end-block ty-contain-cm modeLoaded" spellcheck="false" lang="objc"><div class="CodeMirror cm-s-inner CodeMirror-wrap" lang="objc"><div style="overflow: hidden; position: relative; width: 3px; height: 0px; top: 9.5px; left: 8px;"><textarea autocorrect="off" autocapitalize="off" spellcheck="false" tabindex="0" style="position: absolute; bottom: -1em; padding: 0px; width: 1000px; height: 1em; outline: none;"></textarea></div><div class="CodeMirror-scrollbar-filler" cm-not-content="true"></div><div class="CodeMirror-gutter-filler" cm-not-content="true"></div><div class="CodeMirror-scroll" tabindex="-1"><div class="CodeMirror-sizer" style="margin-left: 0px; margin-bottom: 0px; border-right-width: 0px; padding-right: 0px; padding-bottom: 0px;"><div style="position: relative; top: 0px;"><div class="CodeMirror-lines" role="presentation"><div role="presentation" style="position: relative; outline: none;"><div class="CodeMirror-measure"><pre><span>xxxxxxxxxx</span></pre></div><div class="CodeMirror-measure"></div><div style="position: relative; z-index: 1;"></div><div class="CodeMirror-code" role="presentation" style=""><div class="CodeMirror-activeline" style="position: relative;"><div class="CodeMirror-activeline-background CodeMirror-linebackground"></div><div class="CodeMirror-gutter-background CodeMirror-activeline-gutter" style="left: 0px; width: 0px;"></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span class="cm-operator">-</span> (<span class="cm-variable-3">void</span>)<span class="cm-variable">touchesBegan</span>:(<span class="cm-variable">NSSet</span> <span class="cm-operator">*</span>)<span class="cm-variable">touches</span> <span class="cm-variable">withEvent</span>:(<span class="cm-variable">UIEvent</span> <span class="cm-operator">*</span>)<span class="cm-variable">event</span> {</span></pre></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"> &nbsp; &nbsp;<span class="cm-comment">// 1.自己先处理事件...</span></span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"> &nbsp; &nbsp;<span class="cm-variable">NSLog</span>(<span class="cm-keyword">@</span><span class="cm-string">"do somthing..."</span>);</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"> &nbsp; &nbsp;<span class="cm-comment">// 2.再调用系统的默认做法，再把事件交给下一个响应者处理</span></span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"> &nbsp;  [<span class="cm-keyword">super</span> <span class="cm-variable">touchesBegan</span>:<span class="cm-variable">touches</span> <span class="cm-variable">withEvent</span>:<span class="cm-variable">event</span>];</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;">}</span></pre></div></div></div></div></div><div style="position: absolute; height: 0px; width: 1px; border-bottom: 0px solid transparent; top: 138px;"></div><div class="CodeMirror-gutters" style="display: none; height: 138px;"></div></div></div></pre><p>&nbsp;</p><p>&nbsp;</p><p><strong><span>C.应用圆形按钮 | 只响应部分区域等</span></strong>
<span>代码省略，思路的核心是下面2个方法：</span></p><pre class="md-fences md-end-block ty-contain-cm modeLoaded" spellcheck="false" lang=""><div class="CodeMirror cm-s-inner CodeMirror-wrap" lang=""><div style="overflow: hidden; position: relative; width: 3px; height: 0px; top: 9.5px; left: 8px;"><textarea autocorrect="off" autocapitalize="off" spellcheck="false" tabindex="0" style="position: absolute; bottom: -1em; padding: 0px; width: 1000px; height: 1em; outline: none;"></textarea></div><div class="CodeMirror-scrollbar-filler" cm-not-content="true"></div><div class="CodeMirror-gutter-filler" cm-not-content="true"></div><div class="CodeMirror-scroll" tabindex="-1"><div class="CodeMirror-sizer" style="margin-left: 0px; margin-bottom: 0px; border-right-width: 0px; padding-right: 0px; padding-bottom: 0px;"><div style="position: relative; top: 0px;"><div class="CodeMirror-lines" role="presentation"><div role="presentation" style="position: relative; outline: none;"><div class="CodeMirror-measure"><pre><span>xxxxxxxxxx</span></pre></div><div class="CodeMirror-measure"></div><div style="position: relative; z-index: 1;"></div><div class="CodeMirror-code" role="presentation"><div class="CodeMirror-activeline" style="position: relative;"><div class="CodeMirror-activeline-background CodeMirror-linebackground"></div><div class="CodeMirror-gutter-background CodeMirror-activeline-gutter" style="left: 0px; width: 0px;"></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;">hitTest:withEvent:</span></pre></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;">pointInside:withEvent:</span></pre></div></div></div></div></div><div style="position: absolute; height: 0px; width: 1px; border-bottom: 0px solid transparent; top: 46px;"></div><div class="CodeMirror-gutters" style="display: none; height: 46px;"></div></div></div></pre><h3 id='14-ui卡顿掉帧原因'><span>1.4 UI卡顿&amp;掉帧原因</span></h3><p><strong><span>A.iOS图像显示原理</span></strong>
<img src="image/0001.jpg" referrerpolicy="no-referrer" alt="alt 图像显示原理" title="图像显示原理"></p><p><strong><span>GPU渲染机制</span></strong><span>：CPU 计算好显示内容提交到 GPU，GPU 渲染完成后将渲染结果放入帧缓冲区，随后视频控制器会按照 VSync 信号逐行读取帧缓冲区的数据，再经过数模转换后传递给显示器显示：</span></p><p><span>CPU的工作过程：</span>
<span>1.Layout(UI布局、文本计算)</span>
<span>2.Display(绘制, draw方法)</span>
<span>3.Prepare(图片解编码)</span>
<span>4.Commit(提交位图)</span></p><p><span>GPU的渲染管线处理过程：</span>
<span>1.顶点着色，2.图元装配，3.光栅化，4.片段着色，5.片段处理</span></p><p><a href='https://www.jianshu.com/p/c1bd4e49455d' target='_blank' class='url'>https://www.jianshu.com/p/c1bd4e49455d</a>
<a href='https://www.jianshu.com/p/91d5cf01187c?utm_campaign=hugo' target='_blank' class='url'>https://www.jianshu.com/p/91d5cf01187c?utm_campaign=hugo</a></p><p><span>CPU/GPU向</span><code>Buffer</code><span>(Frame Bufffer帧缓存区)中生成图像，屏幕从</span><code>Buffer</code><span>中取图像，刷新后显示。</span></p><blockquote><p><span>这是一个典型的 </span><strong><span>生产者-消费者</span></strong><span> 模型。</span></p></blockquote><p><span>帧率就是GPU每秒生成的图片(向帧缓存区)，刷新率就是屏幕的刷新率，帧率和刷新率一致就会不卡顿。</span></p><p><strong><span>B.iOS掉帧原因</span></strong>
<img src="image/0002.jpg" referrerpolicy="no-referrer" alt="alt 掉帧原因" title="掉帧原因"></p><p><span>GPU 通常有一个机制叫做垂直同步V-Sync，VSync信号以固定的时间间隔获取帧缓存区的内容并显示到屏幕上，在每一个VSync信号间隔中，CPU负责视图的创建、布局计算、图片解码、文本绘制等内容，紧接着GPU将计算好的内容进行变换、合成、渲染，随后 GPU 会把渲染结果提交到帧缓冲区去。如果在一个 VSync 时间内，CPU 或者 GPU 没有完成内容提交，则那一帧就会被丢弃，等待下一次机会再显示，而这时显示屏会保留之前的内容不变。这就是界面卡顿的原因。</span></p><p><strong><span>C.iOS掉帧优化、滑动优化方案</span></strong>
<a href='https://www.jianshu.com/p/69a122f2bb80' target='_blank' class='url'>https://www.jianshu.com/p/69a122f2bb80</a>
<span>0.先简述为什么iOS掉帧、卡顿</span>
<span>1.对CPU而言:</span>
<span>  对象的创建、调整、销毁、放子线程、重复使用</span>
<span>  预排班（布局计算、文本计算），放子线程(减少主线程压力，提高CPU利用率)</span>
<span>  预渲染（文本的异步绘制、图片的编解码）</span>
<span>2.对GPU而言：</span>
<span>  纹理渲染（减少离屏渲染，少使用透明度，合理使用圆角，阴影）</span>
<span>  视图混合（减少视图层级）</span></p><h3 id='15-ui绘制原理异步绘制'><span>1.5 UI绘制原理&amp;异步绘制</span></h3><p><strong><span>A.UIView的绘制原理（过程）</span></strong>
<img src="image/0003.jpg" referrerpolicy="no-referrer" alt="alt UIView的绘制" title="UIView的绘制"></p><p><span>1.当调用</span><code>UIView</code><span>的</span><code>setNeedsDisplay</code><span>方法时并不会立即绘制视图，它会先调用视图所对应的的</span><code>layer</code><span>的同名方法</span><code>setNeedsDisplay</code><span>方法，相当于在当前视图上打上了个脏标记，然后在当前</span><code>runloop</code><span>将要结束时调用</span><code>CALyer</code><span>的</span><code>display</code><span>方法，最后才会进入绘制流程当中。</span>
<span>2.在调用</span><code>CALyer</code><span>的</span><code>display</code><span>方法时，首先会先判断是否响应</span><code>layer</code><span>的</span><code>displayLayer</code><span>代理方法，如果不响应则进入系统绘制流程，当响应时，则进入异步绘制入口。</span></p><p><strong><span>B.什么是异步绘制？</span></strong></p><p><span>异步绘制，就是可以在子线程把需要绘制的图形，提前在子线程处理好。将准备好的图像数据直接返给主线程使用，这样可以降低主线程的压力。</span>
<span>通过系统的</span><code>[layer.delegate displayLayer:]</code><span>这个入口来实现异步绘制。</span></p><ul><li><span>代理负责生成对应的</span><code>Bitmap</code><span>位图</span></li><li><span>设置该</span><code>Bitmap</code><span>为</span><code>layer.contents</code><span>属性的值</span></li></ul><p><span>其实这个方法的API注释也写的很明白了：</span></p><pre class="md-fences md-end-block ty-contain-cm modeLoaded" spellcheck="false" lang="objc"><div class="CodeMirror cm-s-inner CodeMirror-wrap" lang="objc"><div style="overflow: hidden; position: relative; width: 3px; height: 0px; top: 9.5px; left: 8px;"><textarea autocorrect="off" autocapitalize="off" spellcheck="false" tabindex="0" style="position: absolute; bottom: -1em; padding: 0px; width: 1000px; height: 1em; outline: none;"></textarea></div><div class="CodeMirror-scrollbar-filler" cm-not-content="true"></div><div class="CodeMirror-gutter-filler" cm-not-content="true"></div><div class="CodeMirror-scroll" tabindex="-1"><div class="CodeMirror-sizer" style="margin-left: 0px; margin-bottom: 0px; border-right-width: 0px; padding-right: 0px; padding-bottom: 0px;"><div style="position: relative; top: 0px;"><div class="CodeMirror-lines" role="presentation"><div role="presentation" style="position: relative; outline: none;"><div class="CodeMirror-measure"><pre><span>xxxxxxxxxx</span></pre></div><div class="CodeMirror-measure"></div><div style="position: relative; z-index: 1;"></div><div class="CodeMirror-code" role="presentation"><div class="CodeMirror-activeline" style="position: relative;"><div class="CodeMirror-activeline-background CodeMirror-linebackground"></div><div class="CodeMirror-gutter-background CodeMirror-activeline-gutter" style="left: 0px; width: 0px;"></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span class="cm-comment">/* If defined, called by the default implementation of the -display</span></span></pre></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"> <span class="cm-comment">* method, in which case it should implement the entire display</span></span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"> <span class="cm-comment">* process (typically by setting the `contents' property). */</span></span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span class="cm-operator">-</span> (<span class="cm-variable-3">void</span>)<span class="cm-variable">displayLayer</span>:(<span class="cm-variable">CALayer</span> <span class="cm-operator">*</span>)<span class="cm-variable">layer</span>;</span></pre></div></div></div></div></div><div style="position: absolute; height: 0px; width: 1px; border-bottom: 0px solid transparent; top: 92px;"></div><div class="CodeMirror-gutters" style="display: none; height: 92px;"></div></div></div></pre><p><span>而设置</span><code>layer.contents</code><span>属性，除了</span><code>CGImage</code><span>之外，也可以是</span><code>Core Graphics</code><span>框架绘制的位图</span></p><h3 id='16-ui离屏渲染'><span>1.6 UI离屏渲染</span></h3><p><strong><span>A.什么是离屏渲染</span></strong>
<span>离屏渲染（Off-Screen Rendering）的概念，起源于GPU，它是指GPU在当前屏幕缓冲区以外，开辟新的缓冲区进行渲染操作。与之对应的是 在屏渲染.</span></p><p><em><span>在屏渲染(On-Screen Rendering)，指GPU的渲染操作是在当前显示的屏幕缓冲区中进行</span></em></p><blockquote><p><span>注：从硬件角度来说的解决办法有双重缓存(Double Buffer)，三重缓存(Triple Buffer)</span></p></blockquote><p><strong><span>B.哪些操作会触发离屏渲染？</span></strong>
<span>1.光栅化，</span><code>layer.shouldRasterize = YES</code>
<span>2.遮罩，</span><code>layer.mask</code>
<span>3.圆角，需同时设置</span><code>layer.masksToBounds = YES</code><span>、</span><code>layer.cornerRadius</code><span>大于0</span>
<span>考虑通过 CoreGraphics 绘制裁剪圆角，或者叫美工提供圆角图片</span>
<span>4.阴影，</span><code>layer.shadowXXX</code><span>，如果设置了</span><code>layer.shadowPath</code><span>就不会产生离屏渲染</span></p><p><strong><span>C.为何要避免离屏渲染</span></strong>
<span>触发离屏渲染时，会增加GPU的工作量，可能会造成GPU和CPU的耗时，超过16.7ms，可能会导致视图的卡顿和掉帧。</span></p><p><strong><span>D.优化离屏渲染</span></strong></p><ul><li><span>对于view的圆形边框,如果没有backgroundColor，可以放心使用cornerRadius来做</span></li><li><span>对于所有的阴影，使用</span><code>shadowPath</code><span>来规避离屏渲染</span></li><li><span>应用第三方库</span><code>AsyncDisplayKit</code><span>(Texture)作为主要渲染框架</span></li><li><span>对于图片的圆角，提前美工或者</span><code>CoreGraphics.Kit</code><span>给裁剪好圆角</span></li><li><span>自己管理渲染过程</span></li></ul></div></div>
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